Pneumatic rubber tires are conventionally prepared with a rubber tread which can be a blend of various rubbers which is typically reinforced with carbon black.
In one aspect, tires such as, for example, truck tires and other relatively large tires including relatively heavy duty light truck tires, which are normally expected to be capable of supporting and carrying relatively large vehicular loads, tend to generate a greater internal temperature than comparable passenger vehicle-type tires.
The truck tire treads are typically of a cap/base construction, with the tread cap designed to be ground-contacting and, thus, contain a lug/groove ground-contacting configuration, and with the tread base underlying and supporting the tread cap and positioned between the tread cap and the tire casing. Generally, the tread cap and tread base are a co-extruded component of the tire. The tread base is not intended to normally be ground-contacting and, thus, not normally intended to have the same measure of tread properties as, for example, the tread cap properties of traction and treadwear resistance.
For some truck tire treads, particularly for tire carcasses of a bias ply construction, the tread base is relatively thick, as compared to the thickness of the tread cap, and is often of a thickness of at least 50 percent of the thickness of the associated tread cap, instead of being a relatively thin tread base of a thickness less than about 25 percent of the thickness of the associated tread cap as may be found, for example, in passenger tires. This is in contrast to radial ply tires, such as for example passenger tires and light truck tires, which often have a tread of a cap/base construction where such tread base is often relatively thin, such as being less than 25 percent of the thickness of the associated tread cap.
For such relatively heavy duty tires, heat buildup, evidenced by tire running temperature buildup, or increase, is normally unwanted, although a substantial portion of the heat buildup, or temperature increase, is relatively unavoidable during a running of the tire under load.
However, it remains to be desirable to provide a rubber tire tread for use with tires on various trucks including light to medium trucks, which will generate less heat under typical operational conditions particularly since higher tire running conditions tend to prematurely age the tire rubber and associated tire construction and, thus, tend to shorten the effective life of the tire, which may include the tire casing. The effective life of the tire casing is considered herein to be important because it is desirable to retread the tire casing after the tire tread becomes worn.
On the road, medium truck tires are not only used for the typical, paved road applications for which they are designed, but also on the roads which could be defined as mixed service application, having rocks and gravel present. This type of service causes tread chipping/chunking, therefore ending tire life prematurely and causing customer dissatisfaction. Furthermore it is desirable to improve a chip/chunk resistance of mixed service tires and therefore their lifetime.
In another aspect, it is desired to provide such a tire which can generate less heat under operational conditions, yet have acceptable treadwear and traction (skid resistance).
It is envisioned that such tread would be of a cap/base construction where the tread cap and base are of natural rubber compositions reinforced with carbon black and optionally with a combination of carbon black and silica. A cap/base construction for tire treads is well known to those skilled in such tire tread art. For example, see U.S. Pat. No. 3,157,218 which is intended to be incorporated herein by reference as being illustrative of such tread construction.
Conventionally, truck tire (including bus tires) tread cap rubber compositions are composed predominately of natural rubber (natural cis 1,4-polyisoprene rubber) in order to achieve satisfactory heat durability and cool running characteristics. Other elastomers are often mixed with the natural rubber for such tread caps such as, for example, styrene/butadiene copolymer rubber and sometimes cis 1,4-polybutadiene rubber.
In practice for preparation of tire treads, various rubbers, or elastomers, are typically evaluated, selected and blended for a purpose of achieving desired tire tread properties and particularly a balance of tire tread characteristic properties, mainly, rolling resistance, traction and wear. Typically, however, the chip/chunk performance of such tires is poor, resulting in premature retreading or scrapping of tires.
For various applications utilizing rubber, including applications such as tires and particularly tire treads, sulfur-cured rubber is utilized which contains substantial amounts of reinforcing filler(s). Carbon black is commonly used for such purpose and normally provides or enhances good physical properties for the sulfur-cured rubber. Particulate silica might also sometimes be used for such purpose, particularly if the silica is used in conjunction with a coupling agent. In some cases, a combination of silica and carbon black has been utilized for reinforcing fillers for various rubber products, including treads for tires.
The term “phr” where used herein, and according to conventional practice, refers to “parts of a respective material per 100 parts by weight of rubber, or elastomer”.
In the description of this invention, the terms “rubber” and “elastomer”, where used herein unless otherwise prescribed, are used interchangeably. The terms “rubber composition”, “compounded rubber” and “rubber compound” where used herein unless otherwise prescribed, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients or materials”, and such terms are well known to those having skill in the rubber mixing, or rubber compounding, art.